Ah...I too was weary about the DB's in this game. In past years game they would just sit there and watch as a ball flies right over their head only to land in the hands of a streaking WR. I always thought, they should have just made an attempt to make a play on the ball.
Even on Varisty level in the new Demo DB's are very alert and always attempt to make a play at the ball. My freinds and I have been playing NCAA Football since its release on the original playstation 2 in 2002 and have all agreed that the alertness of the DB's has vastly improved! the only thing I've sensed so far is that HB screens appears to work too well.
Don't take my word for it. Log onto the playstation store via your playstation 3 and download the free demo. Also download the MLB Show 2008 and Tiger Woods 08 demos for free to.